On Fri, Feb 16, 2018 at 3:38 PM, Ben Lewis <benlew...@gmail.com> wrote:
> I have been using dataclasses package in a pet project of mine. I'm sorry > if this issue has already been raised. I came across a situation where I > wanted to use the __post_init__ function to initialise some inherited > fields from a dataclass with frozen=True. The problem is that because it is > frozen, assigning to the field doesn't work. > > There are two workarounds without changing the base class to frozen=False, > which could be in a library. > > 1. Use object.__setattr__, this is ugly and not very user or beginner > friendly. > 2. Extract __post_init__ out into a factory function. Then it also loses > all the advantages of the __post_init__ and InitVar mechanism. > > Both frozen and unfrozen dataclasses should be able to use the same > initialisation mechanism for consistency. Being consistent would ease of > converting an unfrozen dataclass to a frozen one if the only code that > actually modifies the instance is in __post_init__ function. > > I think frozen classes should be able to be mutated during the > __post_init__ call. To implements this a frozen dataclass could have a flag > to says it's not yet fully initialised and the flag would be checked in the > frozen setattr/delattr methods. This flag could be located as a special > attribute on the instance or be in a weak reference dict. > That's a pretty tricky proposal, and one that's been debated on and off for a long time in other contexts. And that flag would somehow have to be part of every instance's state. In general the right way to initialize an immutable/frozen object is not through __init__ but through __new__ -- have you tried that? Also, a small example that demonstrates your need would do wonders to help us understand your use case better. -- --Guido van Rossum (python.org/~guido)
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